[services] Tunnelbroker - Remove SessionItem method.

Summary:
We don't have a database method to remove the SessionItem, because there was no need to remove it explicitly. The item record is removed by the DynamoDB TTL. We don't have any device "logout" for implementing the remove method and removing it explicitly from the tunnelbroker side.

Now we need to remove the SessionItem record from the database explicitly in tests (we'll insert, check and remove).

Test Plan:
Successfully built and run service using yarn run-tunnelbroker-service.
SessionItem record was removed by the SessionID key.

Reviewers: karol-bisztyga, palys-swm, varun

Reviewed By: karol-bisztyga, palys-swm

Subscribers: ashoat, palys-swm, Adrian, atul, karol-bisztyga, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3641